Pumps can be classified into mechanical and non-mechanical varieties. Generally, the conventional mechanical pumps have issues with reliability of the moving pump-components. Electrokinetic pumps, on the other hand, contain no moving parts, making them suitable for a variety of applications, including fluid movement in microanalytical systems. Electroosmotic pumps (EOPs) are one of the most represented class of these pumps, and provide fluid flow due to movement of an electric double layer that forms at the solid-liquid interface. Application of an electric field across a porous membrane structure of an EOP results in a movement of the electric double layer, which results in viscous drag. The viscous drag then causes bulk fluid flow and generation of a net pressure.
Standard EOPs made from a ceramic frit or packed capillaries require over 1 kV to establish the electric fields required for pumping. The EOPs produce fluid pressure capable of valve actuation, but yield no benefits in terms of voltage requirements or overall size. Alternative thin porous substrates have, so far, produced the highest pumping pressures per applied voltage due to high surface-to-volume ratios. A small pore length across a thin porous substrate enables the development of high electric field strength across each pore, thus increasing the pumping efficiency. However, such single membrane pumps have pressure and flow limitations, such that application of a few volts generate pumping pressure of less than 1 PSI; thus, utilization as an actuating power source remains impractical.
To increase the pumping pressure of low-voltage EOPs and thus enable self-contained valve actuation, increased surface area for electric double layer formation is required, and hence increased thickness of the substrate (for example, membrane). However, there is no current solution for an arrangement of membranes and electrodes in an EOP, such that the high pressure may be accomplished at low running voltages without changing the electric field strength across the individual pores. In addition, standard methods (e.g. hydrolyzing metal electrodes) of generating ionic currents within the EOPs have detrimental effects on the pump operation, due to the release of gas during pumping.
The low pressure constraint remains a limiting factor for practical utility of low-voltage EOPs. Still, the need for self-containment in analytical, biomedical, pharmaceutical, environmental, and security monitoring applications remains a great challenge, and battery-driven EOPs may serve to replace the limiting control equipment required to run devices, such as high voltage power or pressure supplies.
Maintaining high electric field strength, while using low running voltages are two conflicting requirements, which are difficult to accomplish through conventional EOPs. Therefore, the EOPs which are capable of generating high pressure using a lower applied voltage that maintain membrane fabrication requirements are desirable. Moreover, the simplicity of the EOP processing also makes EOPs a candidate pressure source for actuation of valves within fluidic systems. However, pressure limitations associated with current low-voltage EOPs makes this practically challenging. Still, the ability to package each valve with its own actuator and power source may solve many current problems associated with miniaturization of standard lab-scale control equipment.